Alloy powders based on titanium, zirconium and hafnium are used in the pyro industry, in the production of electrical igniters, for example in airbags and ignition delay elements, as getter materials in vacuum tubes, in lamps, in vacuum equipment and in gas purifiers. Given the very high standards of reliability required of the aforementioned products, for example airbag igniters, it is desirable to produce the alloy powders reproducibly with consistent properties from one batch to the next, particularly with regard to burning time, ignition temperature, average particle size, particle size distribution and oxidation number. It is also desirable to be able to set the cited properties to specific values from the outset.
The alloy powders can be produced by means of a combined reduction and alloying process. To this end the oxide of titanium (TiO2), zirconium (ZrO2) or hafnium (ZrO2) is reduced together with the powdered alloying elements and a reducing agent such as calcium and/or calcium hydride and/or magnesium and/or barium. The reduction is performed in a closed vessel that is capable of being inerted. The reducing agent(s) is/are generally added in excess. Following the reduction the reducing agent oxides formed are removed by leaching with acid and subsequent washing with water. The oxygen content of the metal alloy powders obtained is between 1 and 5% with this process.
Alternatively the alloy powders based on Ti, Zr, Hf can be obtained from the corresponding metal by hydrogenation and dehydrogenation (HDH process). The corresponding metal is hydrogenated and in this brittle form can then be mechanically comminuted to a powder of the desired fineness. Ultra-pure hydrogen must be used for hydrogenation to prevent damage due to absorption of oxygen and nitrogen. The comminution of the hydrogenated metal to the desired particle size must likewise take place in a pure protective gas atmosphere, for example helium or argon. For the subsequent removal of hydrogen and formation of the alloy, the titanium, zirconium or hafnium metal hydride powder and the metal hydride powder or metal powder to be alloyed is broken down under vacuum at elevated temperature and alloyed at the same time.
One of the disadvantages of alloy powders produced in this way is that they do not have a reproducible burning time, specific surface area, particle size distribution or ignition temperature.